1. Field of the Invention
The present invention relates to lighting devices such as a discharge-lamp lighting device and an LED lighting device, and more particularly, to start and stop control of a control IC, a driver, or another driving circuit for driving a switching element used in a lighting device.
2. Description of the Related Art
Since inverters can easily output a high frequency rectangular signal, they are used in lighting devices for high intensity discharge (HID) lamps. They are also used in direct-current power supply circuits that light LED light sources. In inverters, many switching elements such as power MOS FETs, which can be driven at high frequency, are used, and special analog control ICs are used in many cases to drive these switching elements. In recent years, control ICs for switching elements have become available in the market at inexpensive prices and many of them are highly functional. Therefore, when a lighting device includes a plurality of switching elements, they are operated in units of circuit blocks each including one or a plurality of switching elements and a special control IC therefor. A standard control voltage of about 15 V is supplied to each control IC from a control power supply circuit provided for the lighting device. Power is supplied to the control power supply circuit from the lighting device. For example, a part of the power input to the lighting device, or a part of the direct-current power converted by the lighting device, is used (see FIG. 1 of Japanese Unexamined Patent Application Publication No. Hei-9-55296).
When the plurality of control ICs operate with the common control voltage, they may be activated not in a desired order but in a random order, or they may operate in an unstable manner even if they are activated, causing circuit failure. This is because the operating voltages, especially the minimum operating voltages, of the special control ICs provided for the circuit blocks are different from each other. When the power to the lighting device is turned on, first the control power supply circuit starts up, and the control voltage supplied from the control power supply circuit rises to reach the standard voltage, 15 V, in the start-up process. In this rise time, control ICs in which the control voltages reach the minimum operating voltages earlier are activated earlier. However, since the control ICs are designed according to the functions and characteristics of the corresponding circuit blocks, it is very difficult to make the minimum operating voltages of the control ICs equal. Therefore, it is demanded that, even when many control ICs having different minimum operating voltages are used, they be activated in a desired order.
When the control ICs are stopped when the power supplied to the lighting device is interrupted, the control ICs may stop not in a desired order but in a random order. This is also because the minimum operating voltages of the control ICs are different from each other. In the fall time of the control voltage, control ICs in which the control voltages reach the minimum operating voltages earlier stop operating earlier. In addition, some control ICs continue to output a voltage in malfunction even if the control voltage becomes lower than the minimum operating voltages. This unstable operation of the control ICs and the random-order stopping of the control ICs are considered as causes of circuit failure.
Even when the power is supplied to the control power supply circuit from a special battery, the same events can occur upon turning on the power or at power interruption. In addition, the same events can also occur if the voltage drops due to fluctuation of the external power source or if the remaining battery capacity is low.
FIG. 5 shows the entire configuration of a discharge-lamp lighting device that uses a general inverter. This discharge-lamp lighting device converts the power input from an alternating-current power source 2 to a desired output power and supplies the output power to a discharge lamp 4. As shown in FIG. 5, the lighting device includes a power-factor correction circuit 6, a step-down chopper circuit 8, a full-bridge circuit 10, and a pulse generating circuit 12. The power-factor correction circuit 6 smoothes the input power and corrects the waveform distortion of the input current to remove harmonic current, and includes a high-frequency switching element for removing harmonic current. The step-down chopper circuit 8 makes the output of the power-factor correction circuit 6 constant and includes a special high-frequency switching element. The full-bridge circuit 10 converts the output of the step-down chopper circuit 8 to a rectangular voltage and adjusts the alternating frequency thereof, and is formed of four switching elements connected in a full bridge. The pulse generating circuit 12 generates high-voltage pulses at the start-up, superposes the high-voltage pulses on the rectangular voltage output from the full-bridge circuit 10, and applies the resultant voltage to the discharge lamp 4.
The circuits are respectively provided with control ICs (IC-1 to IC-4) for driving the switching element or elements of the circuits. A control power supply circuit 14 is also provided to supply a control voltage to these control ICs. In the following description, the power-factor correction circuit 6, the step-down chopper circuit 8, the full-bridge circuit 10, and the pulse generating circuit 12, all of which include a switching element or elements, are simply called switching circuits in some cases.
FIG. 6 is a timing chart showing starting and stopping of the control ICs when the power from the alternating-current power source 2 to the conventional discharge-lamp lighting device configured as described above is turned on or interrupted. As shown in FIG. 6, almost at the same time as when the alternating-current power source 2 is turned on, the output (the control voltage) of the control power supply circuit 14 starts to rise and reaches the standard value within a predetermined rise time. When the alternating-current power source 2 is interrupted, the control voltage starts to fall after a predetermined time elapses. This is because electric charge accumulated in capacitors and other components of the control power supply circuit 14 is discharged when the power source 2 is interrupted, and the supply of the control voltage is maintained for the predetermined time. Therefore, as shown in FIG. 6, the operation of the control power supply circuit 14 includes the rise time and the fall time.
In the conventional lighting device, when the control voltage supplied to the control ICs reaches the minimum operating voltages, the control ICs start to operate. In the rise time of the control voltage, the control IC (IC-2) for the step-down chopper circuit 8, which has a low minimum operating voltage, starts operating first. Then, the control IC (IC-3) for the full-bridge circuit 10 and the control IC (IC-4) for the pulse generating circuit 12, which have intermediate minimum operating voltages, start operating almost simultaneously. The control IC (IC-1) for the power-factor correction circuit 6, which has a high minimum operating voltage, starts operating last. The control IC (IC-4) for the pulse generating circuit 12 automatically stops operating because it does not need to operate after the discharge lamp starts up.
The power-factor correction circuit 6, the step-down chopper circuit 8, and the full-bridge circuit 10 continuously operate while the discharge lamp 4 is on. Therefore, the circuits keep operating at least until the power from the alternating-current power source 2 to the lighting device is interrupted. Even after the power from the alternating-current power source 2 is interrupted, the circuits keep operating while the control voltage is maintained. When the control power supply circuit 14 enters the fall time, the control voltage gradually decreases. Therefore, the control IC (IC-1) for the power-factor correction circuit 6, which has a high minimum operating voltage, stops operating first. Since the control IC (IC-3) for the full-bridge circuit 10 has an intermediate minimum operating voltage, it should stop earlier than the control IC (IC-2). However, as shown in FIG. 6, due to malfunction of the control IC (IC-3), the control IC (IC-3) for the full-bridge circuit 10 and the control IC (IC-2) for the step-down chopper circuit 8, which has a low minimum operating voltage, stop operating almost simultaneously. Although the control voltage is lower than the intermediate minimum operating voltage, the control IC (IC-3) for the full-bridge circuit 10 continues to output a voltage and stops almost at the same time as the control IC (IC-2) for the step-down chopper circuit 8.
From the viewpoint of circuit protection, it is desired in the discharge-lamp lighting device of FIG. 5 that the circuits start operating in the order shown, from the alternating-current power source 2 to the discharge lamp 4, and stop operating in the order shown, from the discharge lamp 4 to the alternating-current power source 2. During the rise time and fall time of the control voltage, however, each of the control ICs starts and stops operating at its minimum operating voltage. In addition, some control ICs may operate even at a voltage lower than their minimum operating voltages. Therefore, as described above, the control ICs may start and stop operating in a random order or the control ICs may operate in an unstable manner.
In the above description, the control ICs drive the switching circuits. The same problems occur also when drivers are used to drive the switching circuits.